<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=513138) </details> <!--IssueSummary end--> related to https://gitlab.com/gitlab-org/gitlab/-/issues/461270#note_2013761770 In scope of this issue, we need to Support the **line number selection** + typing `/explain`  * select multiple lines, (in mac shift+click to select multiple lines) * in duo chat box, type /explain frontend: * translate the https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/lib/gitlab/code_owners/validator_spec.rb#L29-38 into the actually code * truncate if string is too long * send the /explain_code request to backend backend: Seems no backend code changes here